In order to enhance safety by suppressing movement of a field of view of a driver, a head up display (HUD) by which vehicle speed and various kinds of alarm displays are projected to a windshield and a projected display image is superimposed on a landscape in front of a vehicle to be displayed has been proposed.
In Japanese Patent Application Publication No. 2015-102695 (Patent Document 1), a technique that “a display device for emitting a display light that expresses an image toward a transparent member to display a virtual image of the image by means of the display light reflected by the transparent member, the display device including: a display configured to emit the display light that expresses the image; and an optical element that is entered by the display light emitted from the display to transmit and emit the entered display light, the optical element emitting the display light toward the transparent member by changing an angle of an optical axis of the emitted light with respect to an optical axis of an incident light, wherein the optical element is allowed to move in parallel, and an optical path of the display light that is emitted from the optical element and reflected by the transparent member changes in accordance with the parallel movement of the optical element” is described.
Further, in Japanese Patent Application Publication No. 2012-83534 (Patent Document 2), a technique of “a transmissive display device including: a display means for forming an image; and a transparent display panel configured to reflect a display light radiated from the display, wherein the display means includes: a main image generating means for generating a main display element, such as a character and a symbol, to be presented to a user; an auxiliary element generating means for generating one or more auxiliary element by changing at least one of a position, luminance, a size, or a color of the main display element; and an image generating means for generating a display image by superimposing one or more of the auxiliary elements onto the main display element” is described.